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Article Design and Fabrication of Flexible Naked-Eye 3D Display Film Element Based on Microstructure

Axiu Cao , Li Xue, Yingfei Pang, Liwei Liu, Hui Pang, Lifang Shi * and Qiling Deng Institute of Optics and Electronics, Chinese Academy of Sciences, Chengdu 610209, China; [email protected] (A.C.); [email protected] (L.X.); [email protected] (Y.P.); [email protected] (L.L.); [email protected] (H.P.); [email protected] (Q.D.) * Correspondence: [email protected]; Tel.: +86-028-8510-1178

 Received: 19 November 2019; Accepted: 7 December 2019; Published: 9 December 2019 

Abstract: The naked-eye three-dimensional (3D) display technology without wearing equipment is an inevitable future development trend. In this paper, the design and fabrication of a flexible naked-eye 3D display film element based on a microstructure have been proposed to achieve a high-resolution 3D display effect. The film element consists of two sets of key microstructures, namely, a microimage array (MIA) and array (MLA). By establishing the basic structural model, the matching relationship between the two groups of microstructures has been studied. Based on 3D graphics software, a 3D object information acquisition model has been proposed to achieve a high-resolution MIA from different viewpoints, recording without crosstalk. In addition, lithography technology has been used to realize the fabrications of the MLA and MIA. Based on nanoimprint technology, a complete integration technology on a flexible film substrate has been formed. Finally, a flexible 3D display film element has been fabricated, which has a light weight and can be curled.

Keywords: naked-eye 3D; microstructure; flexible; film; fabrication

1. Introduction With the development of science and technology, people are hoping to truly restore three-dimensional (3D) information of the object space. As a result, 3D display technology has emerged with the times and has become a research hotspot in the field of image displays [1–6]. As Wheatstone invented the first stereoscopic picture viewer in 1838, the technology of 3D displays has been developed for nearly 200 years [7]. In this development process, head-mounted 3D display technology is very mature in principle and technology [8–11], and there are a large number of commercial products. However, due to the need of wearing equipment, it is always inconvenient. Meanwhile, long-term use depending on the binocular parallax principle will lead to viewing fatigue, and viewers will feel dizzy. Therefore, it is an inevitable trend for the future development of naked-eye 3D display technology without wearing equipment. Research groups have developed a variety of naked-eye 3D display technologies, including the grating 3D display technology of binocular parallax, holographic technology, and integrated imaging technology. Grating 3D display technology uses the principle of binocular parallax to produce a 3D sensation [12,13]. This technology has the advantages of low cost, simple structure, and easy implementation. However, because the left and right parallax images cannot be completely separated, the viewing area of the viewer is limited, and the 3D image can only be viewed in a relatively fixed position, lacking freedom. Therefore, it is only suitable for a single user and small range of motion. By using the interference principle, holographic technology interferes the light wave reflected by the object with the reference light wave, and records it in the form of interference fringes to form

Micromachines 2019, 10, 864; doi:10.3390/mi10120864 www.mdpi.com/journal/micromachines Micromachines 2019, 10, 864 2 of 9 a hologram [14]. When the hologram is illuminated by a coherent light source, the original light wave will be reproduced based on the diffraction principle, to form a realistic 3D image of the original object. However, the production of high-quality optical holograms requires a high-coherence laser, shockproof platform, and precise optical path setting. In addition, the ambient air flow will also affect the successful recording of holograms. Later, with the development of digital holography technology, using charge coupled devices (CCDs) instead of ordinary holographic recording materials to record holograms and using computer simulations instead of optical diffraction to realize object reproduction, the digitization of the whole process of hologram recording, storage, processing, and reproduction can be realized [15]. However, the resolution of digital holography using CCDs to record coherent light waves cannot be compared to that of holographic dry plates, so the resolution of holograms is relatively low, which seriously affects the image clarity. The computer-generated hologram, which combines digital computing with modern optics, encoding the complex amplitude of the object light wave from a computer to computer-generated hologram (CGH), has unique advantages and good flexibility [16]. However, there is a large amount of data information and processing time for 3D objects, so it is necessary to select appropriate algorithms and coding methods to overcome [5,17]. In addition, the spatial light modulator plays a very important role in the experiment of CGH photoelectricity reproduction. It is necessary to overcome the influence of spatial light modulation on the quality of the reconstructed image [18,19]. At present, the CGH is still in the research stage of algorithm optimization to realize a 3D display of large scale and large field of view. It is still early to expect a commercial product based on holographic display devices. Integrated imaging technology is also composed of two basic processes: Recording and reproduction. Unlike holographic technology, the recording and reproduction process do not require the participation of coherent light, which reduces the difficulty of the whole system. This technology was first proposed by Lippmann, a famous French physicist and Nobel Laureate in physics, in 1908 [20]. The microlens units in the microlens array (MLA) are used to record 3D object information from different perspectives to form a microimage array (MIA), and then the recorded MIA is placed on the focal plane of the MLA whose parameters are matched with a microlens in the recording process. The 3D image can be viewed by irradiating with scattered light according to the principle of optical reversibility and the fusion of the human brain. This technology can provide full parallax and a full-color image, without any special equipment, and the viewpoint provided is quasi-continuous. In a certain area, it can be viewed by many people, which has become an important development trend of naked-eye 3D displays. The recording and reconstruction of 3D objects are formed through the interaction of tens of thousands or even hundreds of thousands of microimages. A array can be used to record the MIA. This method requires a large number of expensive and complex camera equipment, and the mechanical error between camera equipment will also affect the final imaging effect [21,22]. The optical recording method [23] uses an MLA to record the MIA, which is easy to be affected by surrounding environmental conditions such as brightness, sensitivity, and uniformity. The experimental operation is difficult, and the adjacent images are prone to crosstalk, resulting in a poor imaging effect of the final MIA. Then, the acquisition of the MIA based on 3D graphics software is proposed [24], which can realize the free crosstalk and high-resolution recording of the MIA. In addition, the display screen is generally used to display the recorded microimage array in 3D image reproduction. At present, the screen resolution of the mainstream high-definition display screen is 1920 1080, and the number × of per inch is 89, i.e., 89 ppi. Compared to the resolution limit of the human eye of 300 ppi, the resolution is still low. The granular distortion effect will be visible at a certain distance. In this paper, based on the integrated imaging technology, a flexible naked-eye 3D display film element based on microstructures has been designed and fabricated, which can achieve the 3D imaging effect with a resolution higher than the human eye resolution limit of 300 ppi, and has the advantages of curling and light weight. The main arrangement of this paper is as follows: The second part describes the structure design and imaging principle of the 3D display film element. The third part presents the Micromachines 2019, 10, 864 3 of 9 Micromachines 2019, 10, x 3 of 9 thestructural preparation design and of integration the MLA and of the acquisition microstructure, of the MIA. and the The final fourth part part presents describes the summary the preparation of the wholeand integration paper. of the microstructure, and the final part presents the summary of the whole paper.

2.2. Structural Structural Design Design and and I Imagingmaging Pri Principlenciple TheThe structure structure of of the the flexible flexible naked naked-eye-eye 3D display 3D display film element film element consists consists of three ofparts, three namely, parts, annamely, MLA, anMIA, MLA and, MIA, flexible and substrate flexible substratematerial, material,as shown as in shownFigure in1a. Figure The MIA1a. Theis imaged MIA is by imaged the MLA by withthe MLA different with viewing different angle viewing information. angle information. The sub-imag The sub-imageses of each imaging of each imagingchannel are channel fused are to fusedform ato 3D form displaya 3D effect.display The eff humanect. The eye human can watch eye can the watch3D image the 3Dof the image object of thein front object of inthem, front as of shown them, inas Figure shown 1b. in Figure1b.

FigureFigure 1. 1. FlexibleFlexible naked naked-eye-eye 3D 3D display display film film element: element: (a (a) )structure structure composition; composition; ( (bb)) imaging imaging pri principle;nciple; ((cc)) viewing viewing angle. angle.

TheThe aperture ( (DD)) of of the the microlens microlens is is the the same same as as the the size size ( (T)) of of the microimage unit, unit, and and the the distancedistance ( (dd)) between between the the MLA MLA and MIA is the eeffectiveffective focal lengthlength ((ff) of thethe microlens.microlens. A A single single microlensmicrolens can can image image the the corresponding corresponding microimage microimage independently, independently, and several several sub sub-images-images are are fused fused toto form form a a 3D 3D effect. effect. According According to to the the theory theory of of Gauss Gauss optics, optics, when when the the distance distance between between the the MLA MLA and and MIAMIA is the focalfocal lengthlength ofof thethe microlens, microlens, the the image image distance distance is is infinite, infinite, which which means means that that light light from from any anyangle angle on the on MIA the is MIA refracted is refracted by the MLAby the and MLA then and emitted then as emitted parallel as light. parallelTherefore, light. Therefore, the number theof numberpixels of of the pixels reconstructed of the reconstructed 3D image is 3D determined image is determined by the number by ofthe MLAs number (n ofn ).MLAsFinally, (n × the n). imagingFinally, × theresolution imaging (Re) resolution of the element (Re) of the can element be calculated can be accordingcalculated toaccording Equation to (1), Equationwhere L(1),is wherethe size L ofis the the sizeMIA. ofL thecan MIA. be obtained L can be by obtained multiplying by multiplying the array number the array (n numbern) of microimages(n × n) of microimages by the size by of the the × sizemicroimage of the microimage unit (T). The unit viewing (T). The angle viewing of the filmangle element of the film (Figure element1c) can (Figure be obtained 1c) can from be Equationobtained from(2). As Equat theion focus (2). display As the modefocus [display25] is used mode in this[25] paper,is used the in 3Dthis depthpaper, range the 3D (∆ depthZ) is determined range (ΔZ) byis determinedEquation (3), by where EquationPI is (3), the where size PI is of the the pixel microimage. size of the microimage. n Re  , n (1) Re = , (1) L L T  =2arctan( ) ,T (2) θ= 2arctan2d ( ), (2) dD 2d Z=2 . (3) P dD ∆Z=I 2 . (3) PI 3. Structural Design of Microlens Array (MLA) and Acquisition of Microimage Array (MIA)

3.1. Structural Design of MLA

Micromachines 2019, 10, 864 4 of 9

3. Structural Design of Microlens Array (MLA) and Acquisition of Microimage Array (MIA)

3.1. Structural Design of MLA Due to the MLA being the key imaging element, the reasonable design of the parameters of the MLA, such as the aperture, , and array number, is related to the integration of the whole element and the quality of the 3D image. For a miniaturized 3D film element, when the microlens is designed with a large aperture or a small number of MIAs, the resolution of images from all perspectives of the 3D image will be very low, which makes the viewing effect worse. In order to meet the requirement of the human eye resolution of 300 ppi, the structural parameters of the microlens are designed as follows: (1) the material of the microlens is a photosensitive adhesive (NOA61), and the is 1.56 in the visible light band; (2) the aperture (D) of the microlens is 80 µm; (3) the curvature radius of the microlens is 47.5 µm; (4) the focal length (f ) of the microlens is 85 µm; (5) the sag height of the microlens is 22 µm; (6) the array number of the microlens is 250 250. Then, the imaging × resolution is calculated as 317 ppi, which is higher than the human-eye resolution limit. The viewing angle (θ) is about 50◦. At this time, the focal length of the microlens is very short, so the distance between the MLA and MIA is 85 µm. Thus, the whole element will reach the thin-film level.

3.2. Acquisition of MIA The acquisition of tens of thousands or even hundreds of thousands of microimages has been carried out based on 3D graphics software. This method does not need complicated and expensive optical equipment, and can also avoid human and mechanical errors in the operation of optical equipment. With the use of computer memory, computer generation technology can generate microimages quickly, accurately, and in large quantities. Based on 3ds MAX software (3ds MAX 2009, San Rafael, CA, USA), a 3D scene was established, as shown in Figure2. The scene contained the Chinese characters “ 光电所” and letters “IOE.” The central 3D coordinates of “光”, “电”, and “所” were (15.59 mm, 6.8 mm, 9.5 mm), (30.79 mm, − 0, 9.5 mm), and (37.88 mm, 6.8 mm, 9.5 mm), respectively. The central 3D coordinates of “I”, “O”, and “E” were (37.35 mm, 6 mm, 13 mm), (23.43 mm, 0, 13 mm), and (12.48 mm, 6 mm, 13 mm), − respectively. The virtual dynamic camera array was established to simulate the image acquisition process of the whole MLA, and the acquisition of the microimage was carried out for different 3D information from far to near. In the image acquisition, the 3D coordinates of the start point of the camera were A (0, 12.5 mm, 0), and the 3D coordinates of the end point were B (0, 7.42 mm, 19.92 mm). − The field of view of the camera was 5◦ and the moving interval was 80 µm, which was matched with the structural parameters of the MLA. Finally, 250 250 microimages were acquired. The pixel number × of the microimage was 40 40, and the pixel size of the microimage (P ) was 2 µm. Therefore, the 3D × I depth range could be calculated as 6.8 mm. In the process of MIA acquisition, the camera captured images of the 3D scene from different perspectives, as shown in Figure3, to record the information at di fferent perspectives. In addition, the obtained microimages are shown in the box in the upper right corner of the corresponding perspective. It can be seen that the microimages captured by the virtual camera have a very high image resolution and perfect image quality. Micromachines 2019, 10, x 4 of 9

Due to the MLA being the key imaging element, the reasonable design of the parameters of the MLA, such as the aperture, focal length, and array number, is related to the integration of the whole element and the quality of the 3D image. For a miniaturized 3D film element, when the microlens is designed with a large aperture or a small number of MIAs, the resolution of images from all perspectives of the 3D image will be very low, which makes the viewing effect worse. In order to meet the requirement of the human eye resolution of 300 ppi, the structural parameters of the microlens are designed as follows: (1) the material of the microlens is a photosensitive adhesive (NOA61), and the refractive index is 1.56 in the visible light band; (2) the aperture (D) of the microlens is 80 μm; (3) the curvature radius of the microlens is 47.5 μm; (4) the focal length (f) of the microlens is 85 μm; (5) the sag height of the microlens is 22 μm; (6) the array number of the microlens is 250 × 250. Then, the imaging resolution is calculated as 317 ppi, which is higher than the human-eye resolution limit. The viewing angle (θ) is about 50°. At this time, the focal length of the microlens is very short, so the distance between the MLA and MIA is 85 μm. Thus, the whole element will reach the thin-film level.

3.2. Acquisition of MIA The acquisition of tens of thousands or even hundreds of thousands of microimages has been carried out based on 3D graphics software. This method does not need complicated and expensive optical equipment, and can also avoid human and mechanical errors in the operation of optical equipment. With the use of computer memory, computer generation technology can generate microimages quickly, accurately, and in large quantities. Based on 3ds MAX software (3ds MAX 2009, San Rafael, CA, USA), a 3D scene was established, as shown in Figure 2. The scene contained the Chinese characters “光电所” and letters “IOE.” The central 3D coordinates of “光”, “电”, and “所” were (15.59 mm, −6.8 mm, 9.5 mm), (30.79 mm, 0, 9.5 mm), and (37.88 mm, 6.8 mm, 9.5 mm), respectively. The central 3D coordinates of “I”, “O”, and “E” were (37.35 mm, −6 mm, 13 mm), (23.43 mm, 0, 13 mm), and (12.48 mm, 6 mm, 13 mm), respectively. The virtual dynamic camera array was established to simulate the image acquisition process of the whole MLA, and the acquisition of the microimage was carried out for different 3D information from far to near. In the image acquisition, the 3D coordinates of the start point of the camera were A (0, −12.5 mm, 0), and the 3D coordinates of the end point were B (0, 7.42 mm, 19.92 mm). The field of view of the camera was 5° and the moving interval was 80 μm, which was matched with the structural parameters of the MLA. Finally, 250 × 250 microimages were acquired. Micromachines 2019, 10, 864 5 of 9 The pixel number of the microimage was 40 × 40, and the pixel size of the microimage (PI) was 2 μm. Therefore, the 3D depth range could be calculated as 6.8 mm.

MicromachinesMicromachines 20192019,, 1010,, xx 55 ofof 99

InIn thethe processprocess ofof MIAMIA acquisition,acquisition, thethe cameracamera capturedcaptured imagesimages ofof thethe 3D3D scenescene fromfrom differentdifferent perspectives,perspectives, asas shownshown inin FigureFigure 3,3, toto recordrecord thethe informationinformation atat differentdifferent perspectives.perspectives. InIn addition,addition, thethe obtained obtained microimages microimages are are shown shown in in the the box box in in the the upper upper right right corner corner of of the the corresponding corresponding perspective. It can be seen that the microimages captured by the virtual camera have a very high perspective. It can be FigureseenFigure that 2. 250 2.the 250 250 microimages× 250 microimages microimages captured acquired acquired by by dynamic dynamicthe virtual camera. camera. camera have a very high imageimage resolutionresolution andand perfectperfect imageimage× qualquality.ity.

所 FigureFigure 3. 3. ImagingImaging fromfrom differentdifferent different perspectives: perspectives: (a)(a) imagingimaging f fromfroromm one one prespectiveprespective of of “ “所””,”,, (b) (b) imaging imaging ffromfroromm one prespective of of “O” “O”,“O”,, (c) (c) imaging imaging f fromfroromm one prespective of of “ “电电””,”,, and and (d) (d) imaging imaging f fromfroromm one one prespectiveprespective of of “I” “I”.“I”..

Furthermore,Furthermore, 250 250 ×× 250250250 microimagesmicroimages microimages capturcaptur capturededed byby thethe cameracamera werewere encodedencoded andand fusedfused accordingaccording × toto the the the arrangement arrangement arrangement of of of the the the MLA. MLA. MLA. The The The MIA MIA MIA was was generatedwas generated generated by computer by by computer computer processing, processing, processing, as shown as as in shown shown Figure in in4. FigureFromFigure the 4.4. FromFrom enlarged thethe enlargedenlarged images ofimagesimages different ofof differentdifferent regions, regions,regions, we can wewe see cancan that seesee each thatthat microimage eacheach microimagemicroimage is diff isiserent differentdifferent with withinformationwith ininformationformation of di ff ofoferent differentdifferent perspectives perspectivesperspectives from from thefrom 3D thethe scene. 3D3D scene.scene.

FigureFigure 4.4. MicroimageMicroimageMicroimage arrayarray array (( (MIA)MIA)MIA) withwith with differentdifferent different enlargedenlarged images images of different didifferentfferent regions. regions.

4.4. PreparationPreparation andand IntegrationIntegration ThereThere areare twotwo keykey microstructuresmicrostructures inin thethe 3D3D displaydisplay element,element, whichwhich areare thethe MLAMLA andand MIA.MIA. TheThe MLAMLA waswas preparedprepared byby photolithographyphotolithography andand thethe hothot meltingmelting method,method, andand thethe preparationpreparation resultsresults areare shownshown inin FigureFigure 5,5, whichwhich showsshows thethe prototypeprototype ofof thethe MLAMLA (Figure(Figure 5a),5a), micromagnifiermicromagnifier ofof thethe microlensmicrolens (Figure(Figure 5b),5b), andand surfacesurface profileprofile ofof thethe microlenmicrolenss (Figure(Figure 5c).5c). TheThe sagsag heightheight ofof thethe microlensmicrolens waswas 21.97 21.97 μm, μm, which which is is consistent consistent with with the the design design result. result. The The MIA MIA was was also also prepared prepared by by .photolithography. TheThe patternpattern waswas preparedprepared onon thethe substratesubstrate materialmaterial byby aa seriesseries ofof processesprocesses suchsuch asas exposure,exposure, developmendevelopment,t, andand etching.etching. TheThe MIAMIA andand enlargedenlarged imagesimages ofof differentdifferent regionsregions ofof thethe MIAMIA areare shownshown inin FigureFigure 6.6. TheThe substratesubstrate materialmaterial waswas aa ppolyethyleneolyethylene terephthalateterephthalate ((PETPET)) filmfilm,, whichwhich hashas characteristicscharacteristics of of high high toughness, toughness, smooth smooth surface, surface, and and good good lightlight transmittance transmittance.. The The pattern pattern was was

Micromachines 2019, 10, 864 6 of 9

4. Preparation and Integration There are two key microstructures in the 3D display element, which are the MLA and MIA. The MLA was prepared by photolithography and the hot melting method, and the preparation results are shown in Figure5, which shows the prototype of the MLA (Figure5a), micromagnifier of the microlens (Figure5b), and surface profile of the microlens (Figure5c). The sag height of the microlens was 21.97 µm, which is consistent with the design result. The MIA was also prepared by photolithography. The pattern was prepared on the substrate material by a series of processes such as exposure, development, and etching. The MIA and enlarged images of different regions of the MIA are shown in Figure6. The substrate material was a polyethylene terephthalate (PET) film, which has Micromachines 2019,, 10,, xx 6 of 9 characteristics of high toughness, smooth surface, and good light transmittance. The pattern was made bymade lithography by lithography with a highwith resolutiona high resolution of minimum of minimum linewidth linewidth of 2 µm, of which 2 μm, is equalwhich to is the equal pixel to sizethe ofpixel the size microimage. of the microimage.

Micromachines 2019, 10, x 6 of 9

made by lithography with a high resolution of minimum linewidth of 2 μm, which is equal to the pixel size of the microimage.

FigureFigure 5.5. Preparation results of microlens: ( a) prototype prototype of microlens microlens array (MLA); ( b)) micromagnifier micromagnifier of microlens; (c) surface profile of microlens. ofof microlens;microlens; ((cc)) surfacesurface profileprofile ofof microlens.microlens. Figure 5. Preparation results of microlens: (a) prototype of microlens array (MLA); (b) micromagnifier of microlens; (c) surface profile of microlens.

FigureFigure 6. 6. PreparationPreparation results results of (a) MIA MIA in in di differentfferent areas: areas: ( b) few microimages microimages of of “I”, “I”,, ((cc)) fewfew microimagesmicroimagesFigure ofof 6. “O”,“O”Preparation,, and andand ( (d d results)) few few microimagesmicroimages of (a) MIA in of differentof “E”.“E”.. areas: (b) few microimages of “I”, (c) few microimages of “O”, and (d) few microimages of “E”. Subsequently,Subsequently, integrationintegration ofof thethe twotwo keykey microstructuresmicrostructures needsneeds toto bebe carriedcarried out.out. DuringDuring thethe Subsequently, integration of the two key microstructures needs to be carried out. During the integration,integration, thethe MLA MLA and and MIA MIA need need to to be be aligned aligned one one by by one one to realizeto realize 3D 3D image image reconstruction. reconstruction.In the In integration, the MLA and MIA need to be aligned one by one to realize 3D image reconstruction. In experiment, nano-imprinting alignment technology was used to achieve alignment integration of the the experiment,the experiment, nano nano-imprinting-imprinting alignment alignment technology technology was was used used to achieveto achieve alignment alignment integration integration of of twothe two microstructures,the microstructures, two microstructures, as shown as show as show inn Figure inn in Figure Figure7. 7. 7.

Figure 7.FigureIntegration 7. Integration of thinof thin film film element: element: (a ()a pouring) pouring of polydimethylsiloxane of polydimethylsiloxane (PDMS) materials; (PDMS) ( materials;b) baking and curing; (c) generation of imprinting mold; (d) photosensitive adhesive (NOA61) dropped (b) baking and curing; (c) generation of imprinting mold; (d) photosensitive adhesive (NOA61) dropped on the prepared MIA; (e) leveling; (f) imprinting and UV curing; (g) mold peeled off. onFigure the prepared 7. Integration MIA; of (e thin) leveling; film element: (f) imprinting (a) pouring and UV of polydimethylsiloxane curing; (g) mold peeled (PDMS) off. materials; (b) baking First,and curing; the imprinting (c) generation mold with of imprinting the structural mold; information (d) photosensitive of the MLA adhesive should be (NOA61) prepared. dropped The on imprintingthe prepared mold MIA; is composed (e) leveling; of polydimethylsiloxane(f) imprintingimprinting andand UVUV (PDMS). curing;curing; The ((g) moldfree energy peeled of off. the interface of the PDMS mold is low and has chemical inertness. Therefore, the mold is easy to be separated when First,the moldthe imprinting is used to carry mold out withthe integration. the structural During information the mold preparation, of the MLA PDMS should stroma andbe prepared. curing The imprintingagent mold were pouredis composed into a clean of polydimethylsiloxane beaker at a volume ratio (PDMS).of 10:1, continuously The free energystirring withof the a glassinterface of the PDMSrod. mold A large is lo numberw and ofhas bubbles chemical were inertness. generated Therefore, in the PDMS the prepolymermold is easy until to be the separated bubbles when disappeared gradually. In addition, the PDMS prepolymer was poured on the prepared MLA, as the mold is used to carry out the integration. During the mold preparation, PDMS stroma and curing shown in Figure 7a. Then, the substrate was placed on the coater at a speed of 250 rpm with a time of agent were20 s, shakingpoured off into the excessa clean PDMS. beaker Subsequently, at a volume it was ratio placed of in10:1, a vacuum continuously oven until stirringall the bubbles with a rod. A disappeared large number for curing, of bubbles as shown were in Figure generated 7b. The baking in the temperature PDMS prepolymerand time were until set as 65 the °C bubbles disappeared gradually. In addition, the PDMS prepolymer was poured on the prepared MLA, as shown in Figure 7a. Then, the substrate was placed on the coater at a speed of 250 rpm with a time of 20 s, shaking off the excess PDMS. Subsequently, it was placed in a vacuum oven until all the bubbles disappeared for curing, as shown in Figure 7b. The baking temperature and time were set as 65 °C

Micromachines 2019, 10, 864 7 of 9

First, the imprinting mold with the structural information of the MLA should be prepared. The imprinting mold is composed of polydimethylsiloxane (PDMS). The free energy of the interface of the PDMS mold is low and has chemical inertness. Therefore, the mold is easy to be separated when theMicromachines mold is used2019, 10 to, x carry out the integration. During the mold preparation, PDMS stroma and curing7 of 9 agent were poured into a clean beaker at a volume ratio of 10:1, continuously stirring with a glass rod. Aand large 4 h, numberrespectively. of bubbles Finally, were the generatedPDMS imprinting in the PDMS mold prepolymerwith negative until structural the bubbles information disappeared of the gradually.MLA on the In surface addition, could the PDMSbe peeled prepolymer off from wasthe pouredMLA, as on shown the prepared in Figure MLA, 7c. as shown in Figure7a. Then,Secondly, the substrate the structural was placed information on the coater of the at MLA a speed should of 250 be rpmimprinted with aon time the ofsurface 20 s, shakingof the MIA off theby using excess the PDMS. imprinting Subsequently, mold to realize it was placedthe integration in a vacuum of the oven MLA until and allMIA. the The bubbles process disappeared flow was Micromachinesforas follows: curing, 2019 as shown, 10, x in Figure7b. The baking temperature and time were set as 65 ◦C and7 4of h,9 respectively.First, the Finally, photosensitive the PDMS adhesiveimprinting (NOA61) mold with was negativedropped structural on the prepared information MIA, of as the shown MLA onin andtheFigure surface4 h, 7d. respectively. After could it be was peeled Finally, leveled o ffthe from(Figure PDMS the 7e), MLA,imprinting the asimpri shown moldnting in with mold Figure negative was7c. used structural to imprint information photosensitive of the MLAadhesive.Secondly, on the During surface the imprinting, structural could be information peeledthe high off-precision from of the the MLA alignmentMLA, should as shown device be imprinted in was Figure used on7c. to the align surface the of the MIA bywith using Secondly, the the microimages imprinting the structural one mold byinformation to one. realize On the theof the integration basis MLA of should alignment, of the be MLA imprinted the and photosensitive MIA. on the The surface process adhesive of flowthe MIA was byasirradiated follows:using the by imprinting moldlight withto realize a wavelength the integration of 365 nmof the until MLA it was and cured, MIA. asThe shown process in Figureflow was 7f. asFinally, follows:First, the the PDMS photosensitive mold was peeled adhesive off (NOA61)(Figure 7g) was to obtain dropped an integrated on the prepared 3D display MIA, film as shown element, in Figureas shownFirst,7d. in Afterthe Figure photosensitive it was 8. leveled adhesive (Figure7 e),(NOA61) the imprinting was dropped mold wason the used prepared to imprint MIA, photosensitive as shown in Figureadhesive.Figure 7d. DuringAfter 8a shows it imprinting,was the leveled 3D effect the (Figure high-precision of the 7e), planar the impri displa alignmentntingy, and mold device the was 3D was useddisplay used to toimprinteffect align can thephotosensitive be microlenses seen from adhesive.withvarious the angles. microimagesDuring Figure imprinting, 8b one shows by the one. thehigh 3D On-precision display the basis alignmenteffect of alignment,after devicebending was the of photosensitive usedthe element. to align theMeanwhile, adhesive microlenses wasthe withirradiatedweight the of microimagesthe by ultraviolet3D display one lightelement by with one. on a wavelengthOnthe flexible the basis substrate of of 365 alignment, nm has until been it the wascharacterized, photosensitive cured, as shown which adhesive inis Figureless than was7f. irradiatedFinally,1 g, as shown the by PDMS ultrain Figurevioletmold 8c, light was reaching peeledwith a the wavelength off lightweight(Figure7 g)of tolevel.365 obtain nm Figure until an integrated9 it shows was cured, the 3D 3D as display effects shown from film in Figure element,different 7f. Finally,asviewing shown the angles. in PDMS Figure mold8. was peeled off (Figure 7g) to obtain an integrated 3D display film element, as shown in Figure 8. Figure 8a shows the 3D effect of the planar display, and the 3D display effect can be seen from various angles. Figure 8b shows the 3D display effect after bending of the element. Meanwhile, the weight of the 3D display element on the flexible substrate has been characterized, which is less than 1 g, as shown in Figure 8c, reaching the lightweight level. Figure 9 shows the 3D effects from different viewing angles.

Figure 8. Integration of thin film film element: ( (a)) planar planar display; display; ( (b) curved display; ( c) weight.

Figure8a shows the 3D e ffect of the planar display,and the 3D display effect can be seen from various angles. Figure8b shows the 3D display e ffect after bending of the element. Meanwhile, the weight of the 3D display element on the flexible substrate has been characterized, which is less than 1 g, as shown in Figure8c, reaching the lightweight level. Figure9 shows the 3D e ffects from different viewing angles.Figure 8. Integration of thin film element: (a) planar display; (b) curved display; (c) weight.

Figure 9. 3D effects from different viewing angles: (a) −15°; (b) −10°; (c) 0°; (d) 5°; (e) 20°.

5. Conclusion In this paper, we propose to design and fabricate a 3D display film element based on microfabrication. The imaging resolution is higher than that of the human eye at 300 ppi. At the same time, the element has the characteristics of miniaturization and light weight, which can be applied to FigureFigure 9.9. 3D eeffectsffects fromfrom didifferentfferent viewingviewing angles:angles: ((aa)) −1515°;(; (b) −1010°;;( (cc)) 0 0°; ;((dd)) 5 5°; ;((ee)) 20 20°. . product packaging, handicrafts, anti-counterfeiting, and other− ◦industries.− ◦ Using◦ the◦ 3D display◦ effect to replace the original two-dimensional image display has a certain market application prospect, and 5. Conclusion Conclusions also lays the technical foundation for wearable display equipment. InIn this this paper, paper, we we propose propose to to design and fabricate fabricate a a 3D 3D display display film film element element based based on on microfabrication.Author Contributions: The imagingConceptualization, resolution A is.C higher.; Formal than analysis, that of A the.C. humanand L.X eye.; Funding at 300 ppi. acquisition,At the same Q.D.; microfabrication.Investigation, L.X.; TheSoftware, imaging Y.P. ;resolution Validation, is H higher.P.; Visualization, than that ofL.L the.; Writing human – originaleye at 300 draft, ppi. A .AtC.; theWriting same – time, the element has the characteristics of miniaturization and light weight, which can be applied time,review the and element editing, has L.L .the and characteristics L.S. of miniaturization and light weight, which can be applied to product packaging, handicrafts, anti-counterfeiting, and other industries. Using the 3D display effect Funding: This research was supported by the National Natural Science Foundation of China (Nos. 61605211, to replace the original two-dimensional image display has a certain market application prospect, and 61905251); The Instrument Development of Chinese Academy of Sciences (No. YJKYYQ20180008); The National alsoR&D lays Program the technical of China foundation(No. 2017YFC0804900); for wearable Sichuan display Science equipment. and Technology Program (No. 2019YJ0014); AuthorYouth Innovation Contributions: Promotion Conceptualization, Association, CAS A.C .and; Formal CAS analysis,“Light of AWest.C. and China” L.X .;Program. Funding The acquisition, authors thank Q.D.; Investigation,their colleagues L. Xfor.; Software,their discussions Y.P.; Validation, and suggestions H.P.; Visualization, to this research. L.L .; Writing – original draft, A.C.; Writing – reviewConflicts and of editing, Interest: L .TheL. and authors L.S. declare no conflict of interest. Funding: This research was supported by the National Natural Science Foundation of China (Nos. 61605211, 61905251); The Instrument Development of Chinese Academy of Sciences (No. YJKYYQ20180008); The National R&D Program of China (No. 2017YFC0804900); Sichuan Science and Technology Program (No. 2019YJ0014); Youth Innovation Promotion Association, CAS and CAS “Light of West China” Program. The authors thank their colleagues for their discussions and suggestions to this research.

Conflicts of Interest: The authors declare no conflict of interest.

Micromachines 2019, 10, 864 8 of 9 to product packaging, handicrafts, anti-counterfeiting, and other industries. Using the 3D display effect to replace the original two-dimensional image display has a certain market application prospect, and also lays the technical foundation for wearable display equipment.

Author Contributions: Conceptualization, A.C.; Formal analysis, A.C. and L.X.; Funding acquisition, Q.D.; Investigation, L.X.; Software, Y.P.; Validation, H.P.; Visualization, L.L.; Writing—original draft, A.C.; Writing—review and editing, L.L. and L.S. Funding: This research was supported by the National Natural Science Foundation of China (Nos. 61605211, 61905251); The Instrument Development of Chinese Academy of Sciences (No. YJKYYQ20180008); The National R&D Program of China (No. 2017YFC0804900); Sichuan Science and Technology Program (No. 2019YJ0014); Youth Innovation Promotion Association, CAS and CAS “Light of West China” Program. The authors thank their colleagues for their discussions and suggestions to this research. Conflicts of Interest: The authors declare no conflict of interest.

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